How to correct a large array in C?

Question

I have a short C program which works fine, until I enter a large number as the first argument. I get Segmentation fault if I use 10000000, but not 1000000. I am pretty certain this is because the array is too big. How can I fix this? I am very new to C.

Here is my code:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int isPrime(int n);

int main(int argc, char **argv) {
    int limit;
    limit = strtol(argv[1], NULL, 0);
    int scur = 0, checkp = (int) (limit / 275);

    if (limit < 2) {
        printf("Invalid limit.\n");
        return 0;
    }

    int primes[limit];
    primes[0] = 2;
    int i, j = 1;
    printf("Generating primes...\r");
    for (i = 3; i < limit; i += 2, scur++) {
        if (scur == checkp) {
            scur = 0;
            printf("\rGenerating primes... (%.2f%%)", (i * 100.0 / limit));
        }
        if (isPrime(i)) {
            primes[j] = i;
            j++;
        }
    }
    printf("\nPrimes below %i generated.\n", limit);
    printf("Writing to file...\n");

    FILE *file;
    file = fopen("primes.txt", "w");
    int k;
    for (k = 0; k < (sizeof(primes) / sizeof(int)); k++) {
        if (primes[k] == 0) break;
        fprintf(file, "%i\n", primes[k]);
    }
    fclose(file);
    printf("Finished.\n");

    return 0;
}

int isPrime(int n) {
    if (n < 2) return 0;
    else if (n == 2) return 1;
    else {
        if (n % 2 == 0) return 0;
        int i;
        for (i = 3; i < ((int) sqrt(n)) +1; i += 2) {
            if (n % i == 0) return 0;
        }
    }
    return 1;
}

at what line does the debugger say the segmentation fault occures? — Lefteris E, Apr 21 '13 at 02:39
You're allocating that array on the stack, which has a finite, small size (1MB, generally). Try using malloc to allocate the array (remember to free). — Thomas, Apr 21 '13 at 02:39
This thread is more C specific http://stackoverflow.com/questions/5746377/allocating-a-large-5000-array — Shafik Yaghmour, Apr 21 '13 at 02:44

score 3 · Accepted Answer · edited Apr 21 '13 at 03:00

3

Replace:

int primes[limit];

With

int *primes = calloc(limit,sizeof(int));

Update the loop boundaries:

for (k = 0; k < limit; k++) {
    if (primes[k] == 0) break;
    fprintf(file, "%i\n", primes[k]);
}

And release memory after use:

free(primes);

edited Apr 21 '13 at 03:00

answered Apr 21 '13 at 02:42

Valeri Atamaniouk

5,125
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score 1 · Answer 2 · answered Apr 21 '13 at 02:39

1

You want to allocate too much memory to the stack here int primes[limit];

Use dynamic allocation instead.

answered Apr 21 '13 at 02:39

score 0 · Answer 3 · answered Apr 21 '13 at 02:42

0

int* primes = (int*) malloc(sizeof(int) * limit);

...

free(primes)

answered Apr 21 '13 at 02:42

Lefteris E

2,806
1
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score 0 · Answer 4 · answered Apr 21 '13 at 02:44

0

Your primes array in main() is allocated on the stack, whick has a fixed size limit.

Try put it on the heap:

int* primes = (int*)malloc(limit * sizeof(int));

And remove it from the heap when done:

free(primes);

answered Apr 21 '13 at 02:44

yzn-pku

1,082
7
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How to correct a large array in C?

4 Answers4